As a technique for reducing iron loss of a grain-oriented electrical steel sheet, there is a technique of subdividing a magnetic domain by introducing a strain into the surface of a ferrite (Patent Document 3). However, in a wound iron core, since strain relief annealing is performed in the manufacturing process thereof, at the time of annealing, the introduced strain is relaxed, and thus the subdivision of the magnetic domain does not become sufficient.
As a method of supplementing this shortcoming, there is a technique of forming a groove in the surface of a ferrite (Patent Documents 1, 2, 4, and 5). In addition, there is a technique of forming a groove in the surface of a ferrite and also forming a crystal grain boundary ranging from a bottom portion of the groove to the rear surface of the ferrite in a sheet thickness direction (Patent Document 6).
A method of forming a groove and a grain boundary has a high improvement effect for iron loss. However, in the technique stated in Patent Document 6, productivity is significantly reduced. This is because the width of the groove is set to be in a range of 30 to 300 μm in order to obtain a desired effect and then, attachment of Sn or the like to the groove and annealing, addition of a strain to the groove, or radiation of laser light, plasma, or the like for heat treatment to the groove, is required for further formation of a crystal grain boundary. That is, it is because it is difficult to perform treatment such as the attachment of Sn, the addition of a strain, or the radiation of laser light in exact conformity with a narrow groove and it is necessary to slow a sheet passing speed extremely, in order to realize them. In Patent Document 6, a method of performing electrolytic etching is given as the method of forming a groove. However, in order to perform the electrolytic etching, it is necessary to perform application of a resist, corrosion treatment using an etching solution, removal of the resist, and cleaning. For this reason, the number of processes and the treating time significantly increase.